As is well known in the art, thick film screen printing techniques are widely practiced for depositing films of various materials in the processing of microelectronics circuits. Often, the film to be deposited is a dielectric composition which is intended to be printed over the substrate and various layers, such as conductors, formed on the substrate. For example, a dielectric film is often employed to isolate one printed conductor runner from an adjacent or subsequently formed conductor runner to prevent shorting between the conductors.
During the screen printing operation, a squeegee is used to force a thick film ink through a screen and onto the substrate, wherein hereinafter substrate will generically refer to the exposed surface of a microelectronics circuit being processed. The screen is typically constructed of stainless steel woven mesh, and has a photolithographically-defined pattern which determines the placement of the ink on the substrate. The inks used in screen printing processes usually are composed of an organic vehicle, glass frit, and active materials such as dielectric materials for insulating films, as noted above, or elemental metals or alloys for conductor films, or semiconductor compounds or alloys for resistor films. The flow characteristics, or rheology, of thick film inks can be generally described as pseudoplastic, in that the shear rate is very low at low stresses, but increases sharply once a sufficient stress is reached. As a result, the squeegee must be appropriately formed and its motion across the screen controlled, in order to maintain a constant angle of contact with the ink so that the force exerted on the ink is constant.
A conventional squeegee 50 known in the prior art is illustrated in FIG. 4. As illustrated, the squeegee 50 includes a base 44 having a blade member 42 affixed to a lower end of the base 44. The blade member 42 is typically molded from neoprene or polyurethane, and has the cross-sectional profile illustrated. Specifically, the blade member 42 generally has a working surface 46a and a trailing surface 46b which have approximately the same surface area, each typically having a width on the order of about 6 millimeters as measured in the direction perpendicular to the edge formed between the surfaces 46a and 46b. In addition, both surfaces 46a and 46b are oriented to be at an angle of about forty-five degrees to the surface of the screen 48, which is suspended above the substrate to be printed. Typical operating speeds for the squeegee 50 illustrated are on the order of about 150 to about 250 millimeters per second.
While squeegees having the configuration of that depicted in FIG. 4 are widely used, pin holes or voids are often formed in the thick film formed by the printing process due to the woven structure of the screen 48 and the flow characteristics of the ink. These voids are highly undesirable because they significantly diminish the electrical characteristics of the thick film, such as the insulating capability of a dielectric thick film layer. Reducing the viscosity of the ink in order to reduce the formation of voids is often not a practical solution to the problem, in that the ink may tend to flow through the screen under the force of gravity alone, and/or the ink may tend to flow on the surface of the substrate, which is unacceptable when precise printing of the thick film is required. While the formation of voids may be reduced by sufficiently slowing the speed of the squeegee 50 as it travels across the screen 48, such a solution is often undesirable because it lowers the through-put of the printing process.
Accordingly, what is needed is an improved squeegee and a method for using such a squeegee, in which the formation of voids is substantially prevented in a thick film deposited by a thick film screen printing process using the squeegee. Preferably, such a squeegee enables desirable through-put levels to be maintained by enabling linear speeds of up to 200 millimeters or more for the squeegee as it travels across the surface of the screen.